1. Field of the Invention
The present invention relates to digital transmission techniques whereby information produced in asynchronous form, for instance by variable-length coding and/or by conditional coding, is intended for transmission through a synchronous digital transmission path.
Such techniques are applied in particular to a transmission of television pictures, videophone picturs, videoconference pictures, and the like.
2. Description of the Prior Art
Coded digital signals are usually transmitted in a digital path having a predetermined bit-rate by characterizing each sample through the use of a word carrying a fixed number of bits. However, when it comes to information requiring a high transmission binary rate such as in the picture information case in particular, there is every advantage in reducing the bit-rate by using a variable-length code in which a word carries fewer bits the more probable it is.
Likewise, the conditional replenishment codes make it possible to reduce the information bit-rate by transmitting only significant information, for instance on overstepping a threshold.
Contribution No. 134 from the CCITT (International Telegraph and Telephone Consultative Commitee) study group XV, August 1982, describes a codec for videoconferencing using 2048 kbit/s transmission and recommends a conditional replenishment method. This method consists essentially of transmitting solely the information that has changed from one picture to the next, where the information is transmitted using a variable-length coding.
A digital signal transmission system based on a variable-length coding has already been disclosed in U.S. Pat. No. 4,049,910. Information produced by an encoder appears at an irregular bit-rate and must be regulated by means of a buffer or "elastic" memory in order to be transmitted in a synchronous digital network. At a reception end, the information received at a synchronous bit-rate in a decoder must be recovered at a rhythm identical to that at which it was delivered. This operation is achieved by means of a buffer memory working usually in symmetry with that of the encoder.
A conventional decoding and synchronizing method such as that described in the aforementioned patent consists of writing in the reception buffer memory the information such as it is received, and of carrying out synchronization and decoding of the information at a buffer memory output. This method does however, display many drawbacks:
As far as line and field video synchronization is concerned, the decoding does not permit easy processing of synchronizing words to wipe out any faults introduced by transmission errors such as loss of line synchronization, or synchronizing word, or extra line.
Moreover, as regards variable-length decoding, if a maximum bit number per sample is equal to 10, as the signal reading rhythm in the buffer memory is 2.5 MHz in accordance with the aforementioned CCITT contribution, it is then necessary either to read the buffer memory with a bit-rate of 25 Mbit/s or to use an asynchronous permutator cumbersome to implement at a rhythm of 2.5 MHz.
What is more, as regards the information content stored in the transmission and reception buffer memories, exact symmetry is required which if not achieved entrains loss of information and extensive deterioration in recovered pictures.